1. Field of the Invention
The present invention relates to power components and more specifically to an optimization of the breakdown voltage of vertical type power components, the rear surface of which is capable of being soldered to a heat sink.
2. Discussion of the Related Art
FIG. 1 very schematically shows a partial cross-section view of a portion of a conventional high-voltage power component in a border region thereof. The component, a portion only of which is illustrated, is formed in a lightly-doped silicon substrate 1. In the present description, it will be considered that this substrate is of type N but, of course, all conductivity types may be inverted. The component is delimited at its periphery by a P-type isolating wall 2 extending from the upper surface to the lower surface of the substrate.
In a current configuration of high-voltage vertical semiconductor components, a P-type layer 3 is formed, continuously or not, on the lower substrate surface and extends to reach the isolating wall. On the upper substrate surface side, a layer 4, also of type P, is present. P-type layer 4, N-type substrate 1, and P-type layer 3 are layers that constitute a high-voltage vertical component, the high breakdown voltage being especially due to the large thickness and to the low doping level of substrate 1. PNP layers 4-1-3 may, for example, form a transistor.
A rear surface metallization M1 is in contact with the entire rear surface of the component and a metallization M2 is connected, directly or indirectly, to layer 4. This connection is direct in the case where a PNP transistor is desired to be formed. In the shown case, where it is desired to form a thyristor with or without a gate, an additional heavily-doped N-type layer 5 is formed to form the cathode of the thyristor in contact with metallization M2. The periphery of layer 4 is spaced apart from isolating wall 2 by a portion of substrate 1 and preferably includes a lightly-doped P-type area 6 (Pxe2x88x92) deeper than region 4.
When a positive voltage is applied between metallizations M1 and M2, the blocking (non-conducting) junction is the junction between substrate 1 and P region 4-6. Around this junction, the breakdown voltage is determined by a so-called space charge area delimited by equipotential surfaces E1L and E1H, shown in dotted lines in the drawing. Equipotential surface E1L indicates the area at the low potential of electrode M2, for example, 0 volts. Equipotential surface E1H designates the area at the high potential of electrode M1, for example, 600 volts.
When the device is reverse biased, that is, when metallization M2 is positively biased with respect to metallization m1, the breakdown voltage is essentially provided by the junction between substrate 1, and on the one hand P layer 3, on the other hand isolating wall 2. The limits of the space charge area, that is, the equipotential surfaces at the low potential and at the high potential, respectively, have been designated by E2L and E2H. For a device to have a high breakdown voltage, the extreme equipotential surfaces must be as spaced apart as possible to avoid reaching the breakdown voltage in the semiconductor (on the order of 20 V/xcexcm). Thus, one of the layers in the vicinity of the junction providing the breakdown voltage must have relatively light doping so that the space charge area can extend widely enough therein.
Independently from the need to provide a sufficient breakdown voltage of the component when high voltages are applied thereacross, leakage current problems are also posed. For various reasons, for example due to contamination of the oxides, the N substrate 1 may be strongly depleted at the surface under an upper insulator layer 7. A population inversion may even be achieved in this surface region. There then appears a channel region ensuring an electric continuity between the external periphery of P region 6 and the internal periphery of isolating wall 2. To avoid such leakage currents, it is known to use a so-called stop-channel region formed of a heavily-doped N-type (N+) region 8 at the surface of substrate 1 between the external periphery of region 6 and the internal periphery of wall 2. Although this is not shown in the cross-section view, area 8 forms a ring that extends over the entire periphery of the involved component. Given its high doping level, N+ ring 8 is not likely to be inverted and thus interrupts any inversion channel that could form at the component surface. To favor the equipotentiality of stop-channel ring 8 and avoid a localized depletion, it is conventional to coat diffused ring 8 with a metallization (not shown). To favor the spreading of the equipotential surfaces, in reverse biasing, it is also known to coat the upper surface of isolating wall 2 with a metallization 9 that partially extends towards the inside above insulator 7, thus forming a field plate.
Thus, with peripheral structures of the type of that in FIG. 1 in which the component is surrounded with an isolating wall, very good breakdown voltage properties are obtained. However, this requires very long diffusions at high temperatures, for example 300 hours at 1280xc2x0 C. Further, these diffusions, once performed, occupy a non-negligible surface that may even be greater than the surface of the component that they delimit.
It has thus been attempted to form peripheral structures with no isolating wall. An example of such a structure is illustrated in FIG. 2A. On the upper surface side, substantially the same elements as in FIG. 1 are shown, except for the fact that there is no lateral P well. An N+-type region 10 is only formed at the vicinity of the periphery. Region 10 will preferably be more distant from the outer edge of P-type region 6 than was stop-channel region 8 of FIG. 1.
On the lower surface side, a structure substantially symmetrical to that of the upper surface is formed. Thus, P-type region 3 is interrupted and is surrounded with a lightly-doped P-type region 12 similar to region 6 on the upper surface side. Also, on the lower surface side, an isolating layer 13 is deposited at the component periphery and metallization M1 is in contact with the sole P+ region and extends slightly above isolating region 12. An N+-type region 14 is also formed at the component periphery.
This type of structure, in addition to the fact that it poses more delicate breakdown voltage problems than in the case of FIG. 1, poses a problem when forming the solderings. Conventionally, a vertical component is entirely soldered by its lower surface on a radiator or heat sink via a land for soldering 20 (FIG. 2B). If the heat sink extends over a surface at least equal to the lower surface of the component, soldering 20 tends to have a greater lateral extension and possibly to continue upwards along the component surface. This upward extension is designated by reference 22. This structure has a double disadvantage. On the one hand, the soldering has a field plate function and tends to have potential E2H extend to N+ region 14 and alter the breakdown voltage. On the other hand, soldering extension 22 forms a short-circuit for the component.
To avoid these problems, a heat sink such as illustrated in FIG. 2C, having an upper surface including a boss 24 of smaller surface area than the land intended to be assembled thereon, tends to be used. Thus, soldering 20, even if it exhibits a slight extension 26, will not act as a field plate tending to spread equipotential surface E2H and will not tend to create a short-circuit either. This solution provides satisfactory results but poses manufacturing problems. Indeed, the size of boss 24 is linked to the chip size. A specific heat sink must thus be provided for each chip dimension, not to mention positioning difficulties.
Another solution also provided to solve this type of problem is to dig into the component periphery and fill the hollowing with a glass. However, the presence of glass poses temperature withstand problems and makes cutting operations more delicate.
Thus, an object of the present invention is to provide a peripheral power component structure enabling obtaining of high voltages.
Another object of the present invention is to provide such a structure that raises no manufacturing difficulties.
Another object of the present invention is to provide such a structure that raises no other secondary problems such as those previously mentioned.
To achieve these and other objects, the present invention provides a power component formed in a silicon substrate of a first conductivity type, the lower and upper surfaces of which respectively include a first and a second region of the second conductivity type that do not extend to the component periphery, a high voltage being capable of existing between the first and second regions and having to be withstood by the junctions between the first and second regions and the substrate. A deep insulating region that does not join the first region is provided at the lower periphery of the component, the lower substrate surface between said deep insulating region being coated with an insulating layer, the height of the deep insulating region being greater than that of a possible soldering upward extension formed during the soldering of the lower surface on a heat sink.
According to an embodiment of the present invention, the power component further includes a wide and shallow groove before the deep insulating region.
According to an embodiment of the present invention, the deep insulating region is formed by first etching close grooves from the lower substrate surface at the periphery thereof, and by then performing a thermal oxidation, so that the ribs between the various grooves are oxidized and the grooves are filled.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.